Geomagnetic field

The geomagnetic field is the magnetic force field that surrounds the Earth. It
is attributed to the combined effects of the planetary rotation and the movement
of molten iron in the Earth's' core.

Compasses, instruments that align themselves with the magnetic poles of the
Earth, have been used in navigation for centuries. Some birds (particularly
migratory species) and other animals rely on the geomagnetic field to
orient themselves.